Bottle thread answer doubt
I tried it myself.  The problem is that not all of the vertices line up perfectly, so you end up with some rogue unwelded verts, like so:



See those little evil guys?  

To fix it, select all verts and choose Mesh->Vertices->Remove Doubles.  Then adjust the threshold until they all nicely merge.



Once all the verts are properly welded your spiral select will work.